Table II.

PPi formation in presence of F1,6BP, Pi, and F6P

After PPi formation catalyzed by PFP, PPi is hydrolysed by H+ PPase, forming a pH gradient. The respective free energy change (ΔG1 and ΔG2) are calculated. In the table, the concentrations of PPi found after the reaction F1,6BP + Pi → PPi + F-6P (reaction 1, ΔG1) reaches equilibrium were calculated using the value of 0.31 for the apparent equilibrium constant (Keq) as previously described (Kubota and Ashihara, 1990). The equilibrium PPi concentrations were calculated using the following equation: [PPi] = [F1,6BP] × [Pi] × Keq/[F6P]. The PPi formed in the presence of 0.6 mm MgCl2 is further hydrolyzed by the H+-PPase in the following reaction: H+cyt + PPi → 2Pi + H+vac (reaction 2, ΔG2). The ΔG° = -5.1 kcal mol-1 was taken from the values measured by de Meis (1984) in the presence of 0.6 mm MgCl2 to calculate the ΔG2.

Conditions Concentrations ΔG
F1,6BP PI F6P PPi formed ΔG1a ΔG2a ΔG1 + 2a
m kcal mol-1
Ab 6×10-4 5×10-3 1×10-3 3.3×10-4 -0.6 -3.9 -4.5
Bc 6×10-4 2×10-3 3×10-5 3.5×10-4 -2.1 -5.0 -7.2
Cc 6×10-4 3×10-5 3×10-5 2.3×10-5 -1.3 -8.4 -9.7
Dc 1×10-4 2×10-3 2×10-2 3.0×10-6 -0.02 -2.2 -2.2
• a The ΔG1 values (kilocalories per mole) for different conditions were calculated from the equation ΔG1 = ΔG° + RT In [PPi] × [F6P]/[F1,6BP] × [Pi] using the concentrations shown in the table, and ΔG2 values were calculated from the equation ΔG2 = ΔG° + RT In [Pi]2 × [H+vac]/[PPi] × [H+cyt], assuming a ΔpH of 2 units. The final concentration of F6P derived from contamination of F1,6BP and its hydrolysis was estimated to be 5% (3 × 10-5 m). This concentration of F6P was used to calculate the concentration of PPi formed in experiments in which extra amounts of F6P were not added to the assay medium (conditions B and C). bThe molar concentrations of F1,6BP, Pi, and F6P are those reported in the literature for plant cell cytosol (Rebeille et al., 1983; Ukaji and Ashihara, 1987; Weiner et al., 1987; Kubota and Ashihara, 1990; Davies et al., 1993; Stitt, 1998). cThe molar concentrations of F1,6BP, Pi, and F6P are those used in Figure 2.